Such inspection systems are known as such in the art, for instance from European patent application Nos. EP 0 527 453 A1, EP 0 559 616 A1 and U.S. Pat. No. 5,329,852 and U.S. Re 35,495. According to these known solutions, one side of the printed sheets to be inspected is drawn by aspiration against a substantially flat surface of a stationary suction box, while the other side is captured by a suitable optical quality control apparatus, including either a line camera for scanning the surface of the sheets during transport thereof by the sheet gripper system or an array camera for taking a snapshot of the surface of the sheets.
Similar solutions are disclosed in European patent applications Nos. EP 0 820 864 A1, EP 0 820 865 A1, EP 1 190 855 A1 and EP 1 231 057 A1, which all make use of a stationary suction box or table having a substantially flat surface for aspirating the sheets during the inspection process.
EP 1 190 855 A1, which corresponds to U.S. Pat. No. 6,772,689 B2, discloses an inspection system wherein a suction roller is further located upstream of the stationary suction table with respect to the direction of displacement of the sheets. This suction roller is driven by a separate drive at a circumferential speed that is lower than the displacement speed of the sheets in order to decelerate, or more exactly pull the sheets before they are aspirated against the surface of the suction table and inspected by the array camera.
Other solutions are known for instance from International applications Nos. WO 97/36813 A1, WO 97/37329 A1 and WO 03/070465 A1. According to these other solutions, the printed sheets are inspected using an array camera while the sheets are drawn by aspiration against a curved surface. Such an inspection device making use of an array camera may be disposed at different locations along the path of a sheet gripper system of a printing or processing press depending on the sheet delivery configuration, as for instance illustrated in European patent application Nos. EP 0 985 548 A1, EP 1 777 184 A1 and International application Nos. WO 2005/102728 A1, WO 2007/060615 A1.
Another solution is disclosed in International application No. WO 02/102595 A1 which makes use of a moveable band running above the surface of a suction box.
The above described known inspection systems are satisfying as long as the sheets are being transported by the sheet gripper system along a well-defined path.
Inspection by means of an array camera requires a substantial amount of space as each sheet to be inspected has to be drawn against a reference surface having dimensions corresponding to those of the sheet during the image acquisition process so that the array camera can take a snapshot of the whole surface of the sheet to be inspected.
On the other hand, inspection by means of a line camera requires less space as the image acquisition process involves scanning successive portions of each sheet to be inspected, which successive portions are combined together to build the image of the whole surface of the sheet to be inspected. This process accordingly involves a relative displacement of the sheets with respect to the line camera, which relative displacement is achieved as a result of the transportation of the sheets past the camera by means of the sheet gripper system.
Due to the fact that inspection by means of a line camera involves a relative displacement of the sheets with respect to the line camera, care should be taken that the sheet is being conveyed in a stable way in front of the line camera throughout the image acquisition process. This can be achieved quite easily as long as the sheets are being transported along a rectilinear path past the line camera up to the trailing edge of the sheets. This typically involves a relatively long and flat sheet conveying path downstream of the line camera, the length of which must at least be equal to the length of the sheets to be inspected.
Such a relatively long and flat sheet conveying path downstream of the line camera is however not available in all printing presses making use of a sheet gripper system for the delivery of printed sheets, which fact is problematic. Indeed, a change in the direction of displacement of the gripper bars holding the leading edge of the sheets before the end of the image acquisition process has the effect of creating undulations along the length of the transported sheets, which undulations prevent the sheets from being properly aspirated against a reference surface and negatively affect the inspection process as the undulations create ripples that become visible on the acquired images and cause inspection errors.
There is therefore a need for an improved inspection system making use of a line camera for inspecting the quality of printed sheets which are transported by a sheet conveyor system comprising at least one sheet gripper system.